1. Field of the Invention
The present invention generally relates to a method for forming a poly-silicon (p-Si) film and, more particularly, to a method using sequential lateral solidification (SLS) by laser irradiation through an optical device (for example, a mask or a micro-slit array) to pattern the laser beam and provide a periodic energy profile of the edge of the laser beam passing through the optical device so as to widen the poly-silicon grains and achieve grain size uniformity.
2. Description of the Prior Art
In semiconductor manufacturing, amorphous silicon (a-Si) thin-film transistors (TFTs) are now widely used in the liquid crystal display (LCD) industry because amorphous silicon films can be deposited on a glass substrate at low temperatures. However, the carrier mobility in an amorphous silicon film is much lower than that in a poly-silicon (p-Si) film, so that conventional amorphous silicon TFT-LCDs exhibit low driving current that limits applications for LCD devices with high integrated circuits. Accordingly, there have been lots of reports on converting low-temperature deposited amorphous silicon films into poly-silicon films using laser crystallization.
Presently, poly-silicon films are gradually used in advanced electronic devices such as solar cells, LCDs and organic light-emitting devices (OLEDs). The quality of a poly-silicon film depends on the size of the poly-silicon grains that form the poly-silicon film. It is thus the greatest challenge to manufacture poly-silicon films having large grains with high throughput.
Grain boundary distribution in the poly-silicon film produced by sequential lateral solidification (SLS) exhibits excellent periodicity. FIG. 1A is a conventional system for forming a poly-silicon film using SLS. The system comprises: a laser generator 11 for generating a laser beam 12 and an optical device 13 disposed in a traveling path of the laser beam 12. The optical device 13 has a plurality of transparent regions 13a and a plurality of opaque regions 13b. The optical device 13 is implemented using a mask or a micro-slit array. Each of the plurality of transparent regions 13a is a stripe region with a width W. The laser beam 12 passing through the transparent regions 13a irradiates an amorphous silicon film 15 on the substrate 14 in back of the optical device 13 so as to melt the amorphous silicon film 15 in the stripe regions 15a with a width W. As the laser beam 12 is removed, the melted amorphous silicon film 15 in the stripe regions 15a starts to solidify and re-crystallize to form laterally grown silicon grains. Primary grain boundaries 16 parallel to a long side of the stripe regions 15a are thus formed at the center of the stripe regions 15a and a poly-silicon film is formed to have crystal grains with a grain length equal to a half of the width W, as shown in FIG. 1B.
In order to enhance the throughput, U.S. Pat. No. 6,908,835 discloses a method for forming a poly-silicon film using sequential lateral solidification (SLS) with two laser irradiations. In U.S. Pat. No. 6,908,835, an optical device is used to pattern the laser beam and thus control the grain length, as shown in FIG. 2A and FIG. 2C.
In FIG. 2A, the optical device 20 comprises a plurality of first stripe-shaped transparent regions 21 and a plurality of second stripe-shaped transparent regions 22 so that an amorphous silicon film (not shown) on a substrate (not shown) in back of the optical device 20 undergoes two laser irradiations while moving relatively to the optical device 20 along X-axis. In FIG. 2B, it is given that each of the first and the second transparent regions 21 and 22 has a width W. The spacing between two adjacent first transparent regions 21 and between two adjacent second transparent regions 22 is S. An offset width OS appears between the first transparent regions and the second transparent regions, where OS≧½ W. Therefore, the distance λ between a first primary grain boundary (corresponding to a central line 211 in the first transparent regions 21) obtained after SLS using the first laser irradiation and a second primary grain boundary (corresponding to a central line 221 in the first transparent regions 22) obtained after SLS using the second laser irradiation is λ=(W+S)/2.
In practical cases, however, the system for forming a poly-silicon film in FIG. 1A can further comprise a projection lens apparatus (not shown) disposed on the traveling path of the laser beam 12 between the substrate 14 and the optical device 13. Given that the projection lens apparatus has an amplification factor of N, the grown poly-silicon film has crystal grains that have a grain length of λ/N. For example, if W=27.51 m, S=7.5 μm and N=5, the grain length of the poly-silicon film is λ/N=[(W+S)/2]/5=3.5 μm, as shown in FIG. 2C.
Using SLS with the aforesaid specially designed optical device, a poly-silicon film with periodically arranged poly-silicon grains is manufactured. The grains have a length of 3˜10 μm and a width of only 0.3˜0.5 μm. Accordingly, by increasing the grain width of the poly-silicon grains, the number of grain boundaries per unit area is further reduced so as to significantly improve the electrical characteristics of the think-film transistors using poly-silicon due to the higher carrier mobility and the higher turn-on current.
U.S. Pat. No. 6,322,625 discloses a method for forming a poly-silicon thin film using sequential lateral solidification (SLS) with two laser irradiations as shown in FIG. 3A to FIG. 3F. In FIG. 3A, an optical device (not shown) is used to define a first chevron-shaped region 31 in the amorphous silicon film 30 on the substrate. A first laser irradiation is performed on the first chevron-shaped region 31 to melt the amorphous silicon film 30, as shown in FIG. B. After the laser beam is removed, the melted amorphous silicon is solidified from the edge to form a poly-silicon region 311 and a primary boundary or a nucleus region 312, as shown in FIG. 3C. Then, in FIG. 3D, the substrate is moved with respect to the optical device so as to define a second chevron-shaped region 32, wherein the second chevron-shaped region 32 comprises the primary boundary or the nucleus region 312. A second laser irradiation is performed on the second chevron-shaped region 32 to melt both the Si film (comprising amorphous silicon and poly-silicon portions) in the second chevron-shaped region 32, as shown in FIG. 3E. After the laser beam is removed, the melted Si film in the second chevron-shaped region 32 is solidified from the edge to form a poly-silicon region 321 and thus a poly-silicon grain 33 with a larger width is formed at the tip of the first chevron-shaped region 31, as shown in FIG. 3F.
However, the aforesaid method is only useful when forming a wider poly-silicon grain at the tip of a chevron-shaped region and tends to cause serious uniformity issues.
Therefore, there exists a need in providing a method for forming a poly-silicon film using sequential lateral solidification (SLS) by laser irradiation through an optical device to pattern the laser beam and provide a periodic energy profile of the edge of the laser beam passing through the optical device so as to widen the poly-silicon grains and achieve grain size uniformity.